If you tie your horse's face to it's chest, maybe you'll get a "prettier" circle. Take the ties off? You've got a horse that has no clue what you want him to do. Forcing a horse's head into a frame by tying it down is not teaching the horse how to carry himself around a circle. All it teaches him is to resent being ridden with the ties and to resist his work. Teach, do not force.
Some exercises to help you are spiral in and outs, going straight and tuning onto a medium (15m diameter) circle and going bach to riding straight, lots of riding and transitions on a 20m diameter circle and serpentines or changes of direction.
I love these kind of exercises, keeps the horse on its toes and listening, not just going round and round on an endless circle!